The invention disclosed herein is useful in several processes used by the pulp and paper industry. During pulping processes, cellulosic fibers must be liberated from their encasing lignin matrix so that they can associate with one another, yielding strength in the final product. This polymer separation can be accomplished by removal of lignin as in chemical pulps, or by maintaining the lignin as in high yield mechanical pulps. During the bleaching process, lignin is removed and the resulting pulp is brightened.
The secondary cell wall of wood, composed of cellulose fibrils, hemicellulose and lignin, imparts physical strength and rigidity to woody plants. The cellulose fibrils are densely packed and surround the cell in regular parallel arrays, or in crisscross layers. These fibrils are held together by a matrix of hemicellulose and lignin.
Cellulose is the most abundant component of woody tissue, comprising 35-45% of the dry weight. Cellulose is an ordered linear polymer of glucose monomers coupled by .beta.-1,4 bonds. The hemicelluloses are branched polymers composed of pentose (5-carbon) monomers, normally xylose and arabinose; and hexose (6-carbon) monomers, consisting of glucose, galactose, mannose and substituted uronic acid.
Lignin is an extremely complex polymer formed by the free radical polymerization of substituted cinnamyl alcohol procursors. Lignin constitutes 15-35% of dry wood weight.
Lignin is highly resistant to biological attack; not a surprising finding considering the complexity and stability of lignin structure. No organism has been demonstrated to grow on lignin as the sole carbon source. The complex lignin polymer, however, is completely degraded by pure cultures of various higher order fungi. For reviews see Higuchi (1982) Experientia 38: 159-166, and Janshekar, H. and Feichter, A. (1983) "Advances in Biochemical Engineering/Biotechnology," A. Fiechter and T. W. Jeffries, Eds., Vol. 27, pp. 119-178, Springer, Berlin; Kirk, T. K. (1984) in "Biochemistry of Microbial Degradation," D. P. Gibson, Ed., pp. 339-437, Marcel Dekker, N.Y. The major degraders of "fully lignified" tissues (lignin &gt;20%) are the basidiomycetes that cause the white-rot type of wood decay. The most extensive physiological investigations of lignin biogradation by white-rot fungi have been conducted with a single member of the family Corticraceae, Phanerochaete chrysosporium Burds.
Although P. chrysosporium is capable of completely degrading lignin, purified lignin will not support its growth. Purified cellulose, however, is a growth nutrient for these fungi. Lignin degradation allows these fungi to expose the cellulose food source contained within the lignin matrix. Under defined laboratory conditions, fungal lignin degradation is not observed during the approximately first 3 days of culture. Subsequently, the culture becomes starved for carbon or nitrogen. Lignin degradation is first observed one or two days later and is maximal at 6 days. The induction of lignin degradation in response to carbon and nitrogen starvation indicates that fungal lignin metabolism is a secondary metabolic event (Keyser, P., Kirk, T. K. and Zeikus, J. G. [1978] J. Bacteriol. 135: 790-797.).
Fungal lignin degradation is commercially impractical for several reasons. The rate of lignin degradation is unacceptably slow since ligninolytic activity must be induced by starvation. Furthermore, fungi metabolize cellulose fibers are their primary food source, resulting in reduced pulp yield and an inferior pulp product.
With regard to the major C--C and C--O--C intersubunit linkages found in lignin, it is important to note that approximately 80% of intersubunit bonds involve linkages to the C.sub..alpha. or C.sub..beta. carbons.
Tien and Kirk have disclosed a ligninase preparation capable of oxidatively cleaving C.sub..alpha. -C.sub..beta. bonds in lignin model compounds (Tien, M. and Kirk, T. K. [1984] Proc. Natl. Acad. Sci. 81: 2280-2284). This preparation displays on an SDS-polyacrylamide gel predominantly one protein with an apparent molecular weight of 42 kilodaltons and several minor bands. Thus the preparation is a mixture of proteins without any means suggested for isolating the dominant protein from the minor bands. Subsequent to the publication of this paper, several scientific papers were published disclosing an inability to isolate the major protein from the mixture. These articles are as follows: Huynh, V-B and Crawford, R. L. (1985) FEMS Microbiology Letters 28: 119-123; Leisola, M. et al. (1985) Lignin Biodegradation Workshop; and Gold, M. H. et al. (1985) Lignin Biodegradation Workshop.
These protein isolations have been done by either ion-exchange chromatography or size exclusion-ion exchange column chromatography. The fractions containing ligninase have been analyzed by isoelectric focusing or SDS-polyacrylamide gel electrophoresis, and have shown multiple proteins. The scientists who performed this work are at the forefront of the lignin enzyme field, as evidenced by their participation in the Lignin Biodegradation Workshop held in Vancouver, BC in 1985.
With this background of prior art failures, the inventors of the subject invention were faced with a seemingly insurmountable problem. The invention disclosed herein has successfully solved the problem by producing a substantially pure preparation, designated rLDM.TM. 6, which is free of other proteins contained in the Tien and Kirk mixture disclosed above.
Advantageously, the preparation of the subject invention, rLDM.TM. 6, possesses desirable properties for use in pulping wood and treating effluent which the Tien and Kirk preparation did not have. Specifically, the Tien and Kirk mixture has a lower specific activity than the rLDM.TM. 6 of the subject invention.
There is a clear need to isolate and identify other enzymes which can be used to catalyze the degradation and modification of lignin. The novel rLDM.TM. of the subject invention are useful for this purpose.
These novel compounds are lignin-degrading enzymes which will not attack cellulose or hemicellulose. The enzymes are immediately active and require no metabolic induction; therefore they overcome the drawbacks of fungi previously mentioned for use in pulp operations.